Can I fit lowlight rear bus seats in a highlight? Are the mounts and fittings the same or close enough?

Nope. and Yes.
The Lowlight seats (top backrest) have one tab in the centre that slots into the body.
The late bay have 2 tabs.

I have both sorts but they are at my yard so I cannot take a photo easily. It will be a week before I am out there again but I will try and remember to get a shot for you.

That did not make sense - The No and Yes thing was - no they don;t fit but yes you could modify with a grinder and welder.